U.S. patent number 4,555,806 [Application Number 06/479,771] was granted by the patent office on 1985-11-26 for system for the automatic establishment of a shortwave telegraphy signal connection.
This patent grant is currently assigned to Rohde & Schwarz GmbH & Co. KG. Invention is credited to Udo Bohler, Gunter Greiner, Peter Islet, Wolf-Ruediger Lange, Klaus Volkheimer, Anton Wessel.
United States Patent |
4,555,806 |
Lange , et al. |
November 26, 1985 |
**Please see images for:
( Certificate of Correction ) ** |
System for the automatic establishment of a shortwave telegraphy
signal connection
Abstract
A system for the automatic establishment of a shortwave
telegraphy signal connection between two transmitter-receiver
stations within a frequency range subdivided into several channels.
Both stations use their receiver to determine objective measurement
values reflecting the transmission conditions over the individual
channels. Active and passive channel analysis of the actual quality
of the signal transmission is determined at each channel. The
transmitter of the calling station is automatically tuned to the
channel which corresponds to the best channel quality stored in its
memory. A predetermined time after transmitting the call signal,
the calling station switches to reception and upon reception of an
acknowledgment signal from the counter station automatically begins
the message transmitting-and-receiving operation. If after a
predetermined time no acknowledgment signal is received, the
transmitter automatically switches to the channel stored in its
memory with the next best channel quality value.
Inventors: |
Lange; Wolf-Ruediger (Dachau,
DE), Wessel; Anton (Munich, DE), Bohler;
Udo (Munich, DE), Volkheimer; Klaus (Grafing,
DE), Greiner; Gunter (Munich, DE), Islet;
Peter (Munich, DE) |
Assignee: |
Rohde & Schwarz GmbH & Co.
KG (Munich, DE)
|
Family
ID: |
6159469 |
Appl.
No.: |
06/479,771 |
Filed: |
March 28, 1983 |
Foreign Application Priority Data
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Mar 27, 1982 [DE] |
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3211325 |
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Current U.S.
Class: |
714/708; 455/62;
455/67.14; 714/43 |
Current CPC
Class: |
H04L
1/20 (20130101); H04B 7/22 (20130101) |
Current International
Class: |
H04L
1/20 (20060101); H04B 7/22 (20060101); H04B
017/00 () |
Field of
Search: |
;455/62,52,65,32,67 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2039409 |
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Feb 1972 |
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DE |
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2042133 |
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Mar 1972 |
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DE |
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2408587 |
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Sep 1974 |
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DE |
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2402562 |
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Jul 1975 |
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DE |
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2537683 |
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Mar 1977 |
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DE |
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2659635 |
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Jul 1978 |
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DE |
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2650823 |
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Nov 1978 |
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DE |
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1328595 |
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Aug 1973 |
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GB |
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1328594 |
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Aug 1973 |
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GB |
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Other References
"Assessing HF Propagation Conditions in Real Time", Defense
Electronics Review, May 1980, pp. 21-22. .
"Soft-Decision Error Control for H.F. Data Transmission", IEE
Proceedings, vol. 127, Pt. F, No. 5, Oct. 1980, pp. 389 &
fol..
|
Primary Examiner: Bookbinder; Marc E.
Attorney, Agent or Firm: Willian Brinks Olds Hofer Gilson
& Lione Ltd.
Claims
We claim:
1. A system for establishing a shortwave telegraphy connection
between two transmitter-receiver stations on one of a plurality of
channels, comprising:
each transmitter-receiver station including a transmitter, a
receiver and an antenna, switchable between the transmitter and the
receiver for selectively adapting the station to be either a
calling station or a receiving station;
each receiver having means to search all of said channels prior to
establishing a shortwave telegraphy connection;
testing means at each said station for measuring and evaluting
signals received by the receiver of the respective station on each
channel to determine objective measurement values of the quality of
the signal transmission on each channel, said testing means
including means to perform a passive channel analysis on each
channel before a shortwave telegraphy connection is established
between the two stations including at least measuring and
evaluating the strength and jitter of the received signals;
said testing means further including means for performing active
channel analysis on the data being transmitted on a particular
channel after a shortwave telegraphy connection is established
between the two stations on that channel to obtain additional
objective measurement values of the signal transmission quality on
that channel;
a memory at each said station for storing the objective measurement
values as to each channel obtained from said testing means at the
respective station of said memory; and
processor means at each station interfaced to said transmitter and
receiver, said testing means, and said memory, for controlling the
operation of each respective station and for automatically
establishing a telegraphy connection between a calling station and
a receiving station, said processor means at said calling station
comprising means for directing the transmitter of said calling
station to transmit a call signal over the channel having the best
objective measurement value stored in said memory of said calling
station, said processor means at said receiving station comprising
means for directing the transmitter of said receiving station to
send an acknowledgment signal over the channel of the call signal
after receipt of said call signal, and said processor means at said
calling station further comprising means for directing the
transmitter of said calling station to transmit signals comprised
of a succession of bits over the channel of the call signal thereby
establishing said telegraphy connection, if the acknowledgment
signal is received, and means for directing the transmitter of said
calling station to transmit a call signal over a next selected one
of said channels if the acknowledgment signal is not received
within a predetermined time after the call signal is
transmitted;
said processor means at each said station further comprising means
to replace the objective measurement values stored in said memory
for the channel on which a telegraphy connection has been
established, with said additional objective measurement values
determined by said testing means through active channel
analysis.
2. The system of claim 1 wherein each of said testing means
comprises soft decision measuring means for evaluating signals
received by the receiver at the station of said testing means.
3. The system of claim 1 wherein each of said testing means
comprises means for determining the error rate of bits within
signals received by the receiver at the station of said processor
means.
4. The system of claim 1 wherein the acknowledgment signal includes
a plurality of the objective measurement values from said memory at
said receiving station.
5. The system of claim 1 wherein said processor means at said
calling station further comprises means to direct said transmitter
at said calling station to transmit the objective measurement
values stored in said memory of said calling station to said
receiving station, and wherein said processor means at said
receiving station further comprises means to direct said
transmitter at said receiving station to transmit the objective
measurement values stored in said memory of said receiving station
to said calling station, when said telegraphy connection has been
established, and wherein said processor means at each station
further comprises means to calculate an optimal sequence of
channels for the best transmission between both stations, and means
to store said optimal sequence in said memories at each
station.
6. The system of claim 1 wherein the call signal includes a
plurality of the objective measurement values stored in said memory
at said calling station.
7. The system of claim 1 wherein the call signal comprises a series
of signal frames, each frame comprising a frame counter word having
a sequence of bits to indicate the position of the particular frame
within the frame series and a plurality of repeating words, each
having a sequence of bits which is repeated in each of the signal
frames.
8. The system of claim 7 wherein said processor means at said
receiving station includes means for evaluating the frame counter
word in the call signal to determine the end of the call signal and
means responsive to said evaluating means for switching the antenna
of said receiving station into connection with the transmitter of
said receiving station at the end of the call signal.
9. The system of claim 8 wherein the repeating words of each frame
of the call signal include a synchronizing word, a sender address,
and a receiver address.
10. The system of claim 9, wherein said testing means at said
receiving station comprises soft decision measuring means for
determining confidence values for the bits in received signals.
11. The system of claim 10 wherein said processor means at said
receiving station further comprises correlation means for
synchronizing the receiver at said receiving station with the
synchronizing word in the call signal.
12. The system of claim 10 wherein said processor means at the
receiving station further comprises means for summing the
confidence values of corresponding bits of the repeating words in
successive frames of the call signal.
13. The system of claim 10 wherein the frame counter word is coded
so that from any frame counter word the preceding and following
frame counter word is determinable through a predefined process of
incrementing, decrementing or shifting.
14. The system of claim 1 wherein said processor means at said
calling station further comprises means to direct the transmitter
at said calling station to transmit the objective measurement
values stored in said memory at said calling station to said
receiving station as part of the call signal, and wherein said
processor at said receiving station further comprises means to
update the objective measurement values stored in the memory at
said receiving station upon receipt of said call signal by
averaging them with the objective measurement values received with
said call signal.
15. A method for automatically establishing at a calling station a
shortwave telegraphy connection between the calling station and a
receiving station on one of a plurality of channels comprising:
(a) searching through the channels with a receiver;
(b) performing a passive channel analysis on each channel, said
analysis consisting at least of measuring the strength and jitter
of the digital signals received by the receiver on each channel to
produce objective measurement values for each channel;
(c) storing the objective measurement values in a memory;
(d) tuning a transmitter to the channel having the best objective
measurement value;
(e) sending a call signal through the transmitter over the channel
to which it is tuned;
(f) waiting a predetermined amount of time for an acknowledgment
signal from the receiving station on the channel used for sending
the call signal and if no signal is received, tuning the
transmitter to a next selected one of said channels and performing
steps (e) and (f) until an acknowledgment signal is received;
(g) commencing message transmitting-and-receiving after receiving
an acknowledgment signal over the channel used by the call signal
and the acknowledgment signal;
(h) performing an active channel analysis on the channel on which
message transmitting-and-receiving has been established, said
active channel analysis consisting of at least measuring the
strength and jitter of the digital signals on said channel to
determine additional objective measurement values of the
transmission quality of said channel; and
(i) replacing the objective measurement values obtained by passive
channel analysis and stored in said memory with the additional
objective measurement values obtained by active channel
analysis.
16. The method of claim 15 wherein said passive channel analysis
includes soft decision measuring of the bits in the received
signals.
17. The method of claim 15 wherein said passive channel analysis
includes determining the error rate of bits in the received
signals.
18. The method of claim 15 wherein the acknowledgment signal
includes objective measurement values for each channel stored in
said memory at the receiving station.
19. The method of claim 18 further comprising:
updating the objective measurement values stored in the memory by
averaging them with the objective measurement values received in
the acknowledgment signal.
20. The method of claim 15 wherein the call signal comprises a
series of signal frames, each frame comprising a frame counter word
indicating the position of the particular frame within the frame
series.
21. The method of claim 20 wherein each frame of the call signal
further comprises a synchronizing word, a sender address and a
receiver address.
22. The method of claim 20 wherein each frame of the call signal
further comprises a plurality of the objective measurement values
stored in said memory at said calling station.
23. A method for automatically establishing a shortwave telegraphy
connection at a receiving station with a calling station on one of
a plurality of channels comprising:
(a) searching through the channels with a receiver;
(b) performing a passive channel analysis on each channel, said
analysis consisting at least of measuring the strength and jitter
of the digital signals received by the receiver on each channel to
produce objective measurement values for each channel;
(c) storing the objective measurement values in a memory;
(d) evaluating the signal received on each channel to determine if
it is a call signal;
(e) if the received signal is a call signal, determining the end of
the call signal;
(f) switching an antenna from connection with the receiver into
connection with a transmitter after the end of the call signal;
and
(g) sending an acknowledgment signal over the channel on which the
call signal was received;
(h) receiving data signals from said calling station on the channel
on which said call signal was received;
(i) performing an active channel analysis on the channel on which
said data signals are received, said active channel analysis
including at least measuring the strength and jitter of the data
signals received to determine additional objective measurement
values of the transmission quality of that channel; and
(j) replacing the objective measurement values stored in memory for
the channel on which the data signals are being received with the
additional objective measurement values for that channel determined
by said active channel analysis.
24. The method of claim 23 wherein the acknowledgment signal
includes the objective measurement values for each channel stored
in said memory at said receiving station.
25. The method of claim 23 wherein said passive channel analysis
includes determining the error rate of bits in the received
signals.
26. The method of claim 23 wherein said passive channel analysis
includes soft decision measuring of a confidence value for the bits
in received signals.
27. The method of claim 26 wherein the call signal comprises a
series of signal frames, each frame comprising a frame counter word
having a sequence of bits to indicate the position of the
particular frame within the series of frames and a plurality of
repeating words, each having a sequence of bits which is repeated
in each of the signal frames.
28. The method of claim 27 wherein the end of the call signal is
determined by evaluating the frame counter word.
29. The method of claim 27 wherein the repeating words of each
frame of the call signal include a synchronizing word.
30. The method of claim 29 further comprising summing the
confidence values of corresponding bits of the repeating words in
successive frames of the call signal.
31. The method of claim 30 further comprising:
synchronizing the receiver with the synchronizing word of the call
signal.
32. The method of claim 27 wherein the frame counter word is coded
so that from any frame counter word the preceding and following
frame counter word is determinable through a predefined process of
incrementing, decrementing or shifting.
33. The method of claim 23 wherein the call signal includes the
objective measurement values for each channel stored in memory at
said calling station.
34. The method of claim 33 further comprising:
updating the objective measurement values stored in memory at said
receiving station by averaging them with said objective measurement
values received with said call signal.
Description
BACKGROUND OF THE INVENTION
This invention relates to a system for the establishment of a short
wave signal connection between two transmitter-receiver stations
within a frequency band subdivided into several channels. For the
establishment of a two-way short wave connection between two
transmitter-receiver stations, various transmission criteria must
be taken into account including, for example, the changing
transmission properties of the ionosphere which depend upon the
time of day or sun spot activity, fading effects, interferences
from other transmitters, etc. In normal radio telephony
communication or telegraphy sign communication, all these criteria
are taken into account by a specially trained signal operator. This
person selects the most favorable transmission channels on the
basis of his experience.
In order to facilitate this connection establishment by an
experienced operator, a relatively expensive system described in
"Assessing HF Propagation Conditions in Real Time," Defense
Electronics Review, May 1980, pages 21-22 may be added to the
transmitter-receiver units at the two stations. This system
determines which of the channels is free and undisturbed by
neighboring transmitters and continuously monitors the propagation
conditions over the ionosphere. A spectrum monitor is used in this
system to search through the entire HF spectrum every 10 seconds.
It thus performs a passive channel analysis to check which channels
in the frequency band are free and which are affected by
interference sources. The results of the passive channel analysis
are presented graphically on the CRT display of the system.
Simultaneously, an auxiliary test transmitter sweeps through the HF
bands as often as every five minutes. A test receiver at the
counter station cooperates with the test transmitter by searching
through the entire frequency band to ascertain at any moment the
propagation conditions over the ionosphere at individual
frequencies over the total frequency band. The frequency at which a
transmission will obtain the highest possible level of reception is
thus determinable. The results of this active channel analysis is
displayed on another CRT. The operator can select clear channels
from the passive analysis display and ascertain from the active
analysis display which free channel has the best reception level.
He can determine the best channel for his use and establish the
desired message connection. This system is relatively complicated
and expensive. In addition to the apparatus for message
transmission, it also requires the accessory devices for active and
passive channel analysis. It is also necessary in this system that
the decision of which channels to use is determined by the operator
through evaluating the CRT displays. This system does not provide
an automatic establishment of short wave connections.
Even if the known system described above automatically linked the
active and passive channel analysis to a suitable microprocessor
and from this automatically determined the actual transmission
properties of the individual channels, an automatic connection
establishment would still not be possible. The known system only
analyzes the high frequency transmission properties. These do not
supply an objective measure of the actual channel quality. Thus,
there is a need for an automatically operated system for the
establishment of short wave message connections such as is known in
the UHF range.
In the UHF frequency range, the transmission criteria which are
analyzed for short wave transmissions do not play any part. All
channels are insured of equally good transmission conditions. Thus,
it is a conventional practice to program the transmitters of the
two stations on predetermined free channels. The transmitter is
first tuned to one of the selected channels. The receiver at the
receiving station constantly searches and synchronizes itself
automatically to the frequency selected by the transmitter. At the
end of the call signal first emitted by the transmitter, the
receiver at the receiving station switches its transmitter
automatically to this channel and transmits to the caller a
corresponding acknowledgment signal. It is then possible to perform
the message transmitting-and-receiving operation. This automatic
synchronization between a receiver and transmitter can be repeated
each time the channel being transmitted is changed. Because of the
special transmission criteria which must be taken into account in
the short wave range, this process for automatic establishment of a
connection in the UHF range is not suitable for short wave
connections. It is an object of the invention, therefore, to
provide a system in which the establishment of short wave
telegraphy sign connections between two transmitter-receiver
stations is fully automatic.
SUMMARY OF THE INVENTION
The present invention is characterized by a testing device which
provides an objective measurement value of the quality of signal
transmission over a channel through the use of one or more tests.
The objective measurement values for each channel thereby
determined are sorted and stored in a table in a memory according
to their quality. The transmitter can then be immediately tuned to
the best channel for the start of a message transmission. The
transmitter at the calling station is automatically tuned first to
the channel which is stored in the memory having the best
transmission quality values. The automatic synchronization of the
receiver at the receiving station with the call signal of the
transmitter and the emission from the receiver at the receiving
station of an acknowledgement signal can then be carried out fully
automatically in a known manner. A predetermined time after sending
the call signal, the calling station automatically switches to
reception and upon reception of an acknowledgment signal from the
receiving station, the message transmitting-and-receiving operation
is automatically begun. If after waiting a second predetermined
time, no acknowledgment signal is received, the transmitter at the
calling station is automatically switched to the channel stored in
the memory having the next best channel quality value. This process
continues until a connection is made.
In this system of the present invention, the actual quality of the
signal transmission is determined through an analysis of the
demodulated received signal. Thereby, the actual quality with which
a signal can be received when transmitted over a certain channel is
obtained. The knowledge of this actual transmission quality enables
the provision of the fully automatic connection establishment. This
provides the greatest probability for most rapidly making a
possible connection at the best channel.
According to the present invention, additional transmitters or
receivers are not necessary. The system only requires the use of
the transmitters and receivers which are needed in any case for the
establishment of a message connection. Thus, the total expense of
the system is reduced. All that is needed is a few additional
control devices.
The determination of the actual quality of the signal transmissions
can be carried out in various ways. The quality determination can
be carried out either before the demodulation or after the
demodulation. It has proved to be especially advantageous in making
the quality determination to evaluate the soft decision
information, the reception signal strength and the jitter. The
reception signal strength can be determined in a known manner. Soft
decision decoding is described in "Soft Decision Error Control for
h.f. Data Transmission" IEE Proceedings, Vol. 127, Pt., F, No. 5,
October 1980. In soft decision decoding, each bit in the
transmitted signal is examined for whether it is above or below the
decision threshold as in hard decision decoding, however, there is
also a qualitative determination of the confidence value of each
bit. Another test for determining the quality of the signal lies in
determining in a known manner the jitter of the received
signal.
In the passive channel analysis of the present invention, not only
is it determined whether the channel is occupied or free, but
accidentally present and received telegraphy signals which can come
from other transmitters are examined in each channel. The
evaluation can be done using soft decision decoding or jitter
analysis or both simultaneously. This allows an objective
measurement value to be produced for each channel by a passive
channel analysis, making possible a fully automatic establishment
of a connection.
The call signals sent out by the transmitters are preferably coded
in such a way that when the receiver decodes the signal it can
directly derive from the call signal when the call signal is
finished. Therefore, the receiving station can be switched over
from reception to transmission of the acknowledgment signal at the
appropriate time. This is important when, for example, the call
signal is only poorly received or when the end of the call signal
is not received. The receiver can automatically determine from the
received call signal the actual point in time at which the
acknowledgment signal should be emitted so that it will coincide
with the time when the receiver at the calling station is switched
for reception. There are many possibilities for the coding of the
call signal. An especially simple and advantageous possibility is
to subdivide the call signal into individual frame sections. Each
frame would be transmitted in a constantly repeating pattern of
signal parts which would include segments such as a synchronizing
word, addresses and a status word. A frame counter word would be
included to directly identify the position of the particular frame
within the series of successive frames. The receiver, after
decoding the call signal, can determine from the individual frame
counter words how many frames remain to be received until the end
of the call signal. By example, if it is ascertained that the fifth
from the last frame of a frame series of 32 frames has been
received and if thereafter the reception of the next following four
last frames is interfered with, it is possible for the receiver to
establish that after a time of five frames the end of the call
signal has been reached. After waiting for a certain security time
beyond the end of the call signal, the acknowledgment signal can be
sent out.
For a fully automatic connection establishment, it is especially
important that all received words in the call signal are fully
evaluated, especially if from these received call signal words the
end of the call signal is to be determined exactly. According to a
further development in the invention, it has proved advantageous to
provide a process to improve the dependability of the recognition
of the received bits and the call signal. This process is based
upon the results of the soft decision decoding of the receptions,
the reception signal strength and the jitter. In addition,
according to the invention, there is carried out a weighted
addition of the individual bits which correspond to one another in
words repeated in successive frames. This process can be used not
only in the case of the corresponding bits from the repeated words
in successively following frames, for example, the synchronizing
word, address word or the like, but may also be used with
corresponding special coding of the frame counter word.
After a message connection has been established, further active
channel analysis can supplement and improve the results
corresponding to the channel qualities stored in the memory. This
would also proceed fully automatically.
This invention advantageously provides a system which fully
automatically completes a short wave telegraphy signal or other
data transfer connection with a low cost system. The system of the
invention operates in the establishment of a connection for
transmitted telegraphy signals or other data signals but can also
be switched after establishment of the connection to other types of
modulation, for example, voice radio.
The invention itself, together with further objects and advantages,
will best be understood by reference to the following detailed
description taken in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of one transmitter-receiver station
for use in a system of the present invention; and
FIG. 2 is a representation of call signals used in the system of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the schematic diagram of the apparatus which would be
present at two separate stations Nos. 1 and 2. Each station
includes a transmitter 3 tunable to various channels of a broad
frequency band and a receiver 4 which is also tunable to selected
channels of a broad frequency band and which can execute a search
in this entire frequency band. A common antenna 5 is switchable
between connection with the transmitter 3 and connection with the
receiver 4. The receiver 4 includes a demodulator 6 whose output is
connected into a testing device 7 which determines the actual
quality of the received telegraphy signs. The preferred testing
device 7 includes a soft decision measuring device 8, a jitter
measuring device 9 and a level measuring device 13. The soft
decision measuring device 8 makes a decision as to each received
signed bit whether the bit is "high" or "low", "minus 1" or "plus
1", "mark" or "space" and dispenses through its output a confidence
value indicating the particular quality or dependability of this
yes-no decision. The results of the soft decision measuring device
8 provide the best measure of channel transmission quality. The
jitter measuring device 9 also gives information data about the
quality of the received signals. The jitter measuring device 9
indicates how strongly the received telegraphy signals are
distorted, in other words, how severely they fluctuate. The level
measuring device 13 indicates the amplitude or strength of the high
frequency signal received on the channel. The information data thus
obtained from the testing device 7 about the quality of the
received telegraphy signals are stored in a section of a memory 10,
hereinafter referred to as channel memory 10, reserved for this
channel quality data. Each of the individual devices at each
station are controlled by a central processor 12. The central
processor 12 is programmed to operate these stations as described
in the following details.
Before beginning the establishment of a connection between stations
1 and 2, each receiver 4 executes a search through the entire
frequency band and carries out a passive channel analysis as it
does so. For each channel, the level measuring device 13 measures
the magnitude of the high frequency level received on the channel.
This would include the level of the transmission of a foreign as
well as the transmitter of its counter station which is received on
this channel. It would also include any noise level received.
Simultaneously, soft decision 8 and jitter 9 measuring devices
evaluate the demodulated output signal of telegraphy signals
accidentally received on this channel from foreign sources. A great
amount of jitter of a signal accidentally received in this channel,
means that this channel is not suited for a connection. The poor
quality of the foreign signal received by the channel is
recognized. A similar statement can be made for the soft decision
decoding. Here as well, on finding qualitatively poor signals, it
is perceived that this channel is not suited for transmission.
The bandwidth of a channel is generally about 3 kHz. However, only
about 100 Hz is necessary for transmitting telegraphy signals. It
would therefore be possible to treat the 100 Hz subchannels as
channels and thereby obtain a greater number of channels to choose
from and to analyze. By increasing the number of available channels
the probability of obtaining the best transmission connection is
improved.
In active and passive analysis, besides determining an objective
measurement value for the signal quality by using soft decision and
jitter after the demodulation, the reception level before
demodulation can also be taken into consideration. From this, it is
also possible to draw a conclusion as to the channel quality. Thus,
for each channel, there is ascertained the momentary transmission
quality and the objective measurement values which have been
obtained are each stored in the channel memory 10. The channels are
arranged in order in a table within channel memory 10 according to
their quality. Any one or more of the three tests in the testing
device 7 may be used to measure quality. When all three are used,
the objective measurement value of the transmission quality at each
channel is identified by a single quality number in which the first
place corresponds to the soft decision results. The second place
corresponds to the jitter results and the third place corresponds
to the level measuring results. Thus, soft decision provides the
primary determinant of quality and the other results distinguish
between channels where the soft decision results are the same.
A fourth test whose results could be used in providing an objective
measurement value is determination of the error rate of bits in the
received signals. This test could be performed by the central
processor 12. The test is performed in a known manner, as is done
in telex receivers, for instance.
When a message is to be sent between station 1 and station 2, a
start command will be input into the control processor 12 and then
the processor 12 will tune the transmitter 3 to the channel which
is stored on top of the table within the memory 10. This will be
the channel which has registered the best quality. The transmitter
3 then emits for a predetermined time a call signal. The receiver 4
at the receiving station 2 is constantly executing a search over
all of the channels and should receive the call signal. The
duration of the emission of the call signal is determined according
to the time that the receiver 4 at the receiving station 2 needs
for a complete search of the entire frequency band. The call signal
consists of several successive signal frame sections which each
contain a synchronizing word 16. The signal also contains
information which establishes the end of the call signal. If the
receiver 4 of receiving station 2 actually receives the selected
call signal from the transmitter 3 of station 1, then an evaluating
program 11 within the central processor 12 at the receiving station
2 operates to determine the end of the call signal.
The transmitter 3 at the receiving station 2 is automatically tuned
to the channel on which the call signal was received and an
acknowledgment signal is transmitted. At the calling station 1
after the end of the call signal, the processor 12 switches the
antenna 5 into connection with the receiver 4 for reception of the
selected channel. If this receiver 4 receives the acknowledgment
signal with sufficient quality, the normal
transmitting-and-receiving operation between stations 1 and 2 can
be performed in the known manner. The amount of time for which the
receiver 4 at the calling station 1 waits for reception of the
acknowledgment signal is determined according to the time it takes
for the acknowledgment signal to be transmitted by the receiving
station.
If the receiver 4 at the receiving station 2 does not receive the
call signal on the channel which is first tried, for example,
because momentary poor transmission conditions exist on this
channel frequency between stations 1 and 2, then the calling
station 1 at the end of its call signal will not receive an
acknowledgment signal. The transmitter 3 at calling station 1 will
then automatically switch to the next best channel stored in the
memory 10. This would be the best channel at which an attempt at
establishing a connection was not previously unsuccessful. The
calling station 1 will try again to contact receiving station 2 in
the same manner as before. The attempt to establish a connection
will continue through the third best channel, fourth best channel,
etc. until an acknowledgment signal is received.
After a message connection has been established in this manner, an
active channel analysis is begun on the selected channel. Each
receiver 4 stores the reception level values obtained from the
level measuring device 13. These are stored together with the
values obtained over the jitter measuring device 9 and the soft
decision measuring device 8 in the channel memory 10. These quality
values for the transmission of telegraphy signals are stored
preferably with the clock time and location data for the particular
receiving station for future analysis. In this manner, the
objective measurement values of the passive channel analysis are
updated. The new values obtained through the active analysis can be
averaged into the old values or they may be used to replace the old
values. Thereby over a relatively long period of time after the
establishment of several different connections between the two
stations on different channels by using this active channel
analysis the results stored in the memory 10 concerning the quality
of the individual channels are slowly improved. At each new call,
the stations have available improved results in regards to which is
the most favorable channel at that particular moment. Message
connections between the two stations can be built up an unlimited
number of times fully automatically and in each case on the most
favorable channel.
The results available to the stations can be improved even further
according to an additional feature of the present invention. The
objective measurement values stored at one station can be sent to
the other station by including the sorted objective measurement
values within the acknowledgment signal. Thus, the objective
measurement values stored in memory 10 of calling station 1 can be
revised to obtain a more accurate listing of which channels are the
best for transmission between the two stations. The revision of the
values can be accomplished through averaging the new values into
the old values or by replacing the old values with the new values.
Carrying this approach a little further, during the
transmitting-and-receiving operation between two stations the
objective measurement values stored in the channel memories 10 of
each station may be exchanged with one another. The optimal
sequence for the two stations can then be calculated by the central
processor 12 and stored in the channel memory 10 of each
station.
When using an active channel analysis, the measurement time is
determined according to the type of information being transmitted.
If only short burst signals are transmitted, then the measuring
time for active channel analysis and the determination of the
momentary channel quality is relatively short. For continuous
information transmissions, however, a relatively longer measuring
time is provided for measuring the mean channel quality.
In a system of the type of this invention, it is important that
even under poor transmission conditions the end of the call signal
is established with certainty. This is necessary so that the two
stations will rapidly and surely be synchronized. Therefore, a call
signal as illustrated in FIG. 2 is used. FIG. 2 shows the last
three frames 15 of such a call signal composed of thirty-two
frames. The length of the call signal is governed according to the
time in which it takes the receiver 4 of the receiving station to
complete a search run. This assures that for each search run, the
receiver 4 can actually search all of the channels and still
confirm the reception of a call signal. Each frame 15 consists of a
synchronizing word 16, a frame counter word 17, a receiver address
18, a sender address 19 and a status word 20. It would also be
possible to include some of the objective measurement values within
the call signal. Each word is made up of a number of bits. The
synchronizing word 16, the receiver address 18, the sender address
19 and the status word 20 are repeating words. They are alike for
each frame 15. The frame counter words 17 differ in successive
frames. The frame counter word 17 indicates in which place in the
series of frames that the particular frame 15 is located within the
call signal. In a call signal consisting of thirty-two frames, the
frame counter word 17 of the last frame is, for example, "0", the
preceeding one is "1", etc.
The evaluating program 11 in the central processor 12 tests the
synchronizing words 16 of a received call signal through a
correlation process. By this process the synchronizing word 16 is
compared with a reference word to determine whether they are equal
or nearly equal. If so, frame synchronization has been achieved.
After the frame synchronization, the other words of the frame can
be evaluated. If, for example, the third from last frame 15 is
evaluted and it is ascertained by the evaluating program 11 that
the frame counter word 17 is a "2", then it can automatically be
determined by the evaluating program 11 that after two further
frames "1" and "0", the end of the call signal is reached. That
will be the time when the receiver 4 at the calling station 1 will
be switched to reception. Therefore, that is when the
acknowledgment signal will be sent from the transmitter 3 at the
receiving station 2. Once a frame synchronization is achieved and
the actual number of the frame is identified, the call signal can
be interfered with without upsetting the exact synchronization
between the stations.
In short wave connections, successive bits are often severely
interfered with and then seconds later the connection can be
substantially better. This is caused from fading phenomena and the
like. This could cause synchronization to be delayed or rendered
impossible. According to a further development of the invention,
further steps are taken to improve the recognition of received call
signals. The soft decision data and the jitter data obtained in the
testing device 7 are used to contribute to the evaluation of the
call signal. According to an especially advantageous further
development of the invention, in addition to this information a
separate weighted addition process further improves the probability
of faultless sign recognition. For this purpose, the soft decision
measuring device 8 further provides its output to an additional
area in a memory, which we will call the quality memory 21, in
which the confidence values of like received bits are summed.
In the quality memory 21, each bit is not simply evaluated as "-1"
or "+1", but as a certain negative or positive number. The sign of
each number provides the message content, as in a "hard" decision.
The absolute value of the number is a measure of the quality.
Suppose, that through the soft decision measuring device 8 that for
the last three frames 15', 15" and 15'" shown in FIG. 2 the
synchronizing word 16 is a four bit word with the following
values:
Synchronizing Word Frame 15': +20, +2, -30, -30
Synchronizing Word Frame 15": +30, -30, +30, -30
Synchronizing Word Frame 15'": +10, +1, +10, -30
Correct Synchronizing Word: +, -, +, -
From this it has been determined that in each case the second and
third bit of the synchronizing word has been disturbed. Only
counting the pluses and minuses determined by a hard decision would
yield the bit sequence +, +, +, -, which would be wrong. By adding
up the successively found values, however, there is yielded +60,
-27, +10, -90. From this, the correct message can be perceived. In
this manner, the weighted addition of the successive bit values
into the quality memory 21, substantially improves the probability
of a correct sign recognition.
The weighted addition is applicable only to the repeating words,
those being the synchronizing word 16, the receiver address 18, the
sender address 19 and the status word 20. Suitable modifications
may be made so that the weighted addition method can be used for
the unlike sign sections as well. A suitable coding of the unlike
sign sections needs to be made so that the successive sections can
be easily made equal. It is merely necessary to choose for the
differing sign words a code which permits determination from a
given arbitrary counter state, the previous and successive counter
state, without it being necessary to know the actual value of the
counter state. In other words, the preceding and following word
should be determinable by a predefined process of some combination
of incrementing, decrementing and/or shifting. A simple example is
the so called shift code with the following properties:
______________________________________ 4 0 0 0 0 1 3 0 0 0 1 0 2 0
0 1 0 0 1 0 1 0 0 0 0 1 0 0 0 0
______________________________________
In this code, successive binary numbers are either raised or
lowered by one place in order to achieve equality with the
preceding or following number. If the frame counter words 17 are
coded according to this shift code, then application of this
shifting principle in successive frames can establish bit equality
in each frame. After which the weighted addition can be applied for
improvement of the sign recognition. To accomplish this, the
evaluating program 11 feeds successive frame counter words 17 into
the quality memory 21 shifted by one so that the individual values
can be added up in their storage places. In the equalizing of the
frame counter word 17, the number of necessary shifts should be
directly equivalent to the number of frames received. Therefore,
the weighted addition method can be used on all the bit words of
each frame to improve the sign recognition for the total content of
the frames.
Of course, it should be understood that various changes and
modifications to the preferred embodiments described above will be
apparent to those skilled in the art. It is therefore intended that
such changes and modifications be covered by the following
claims.
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